1. Field of the Invention
The invention relates to a transfer type ink jet recording method and apparatus in which an ink image is formed on a transfer medium and then transferred to a recording medium to thereby obtain an ink image on the recording medium.
2. Description of the Related Art
A transfer type ink jet printer has the advantage that ink jet nozzles are free from clogging due to unintended contacts between a recording head and a recording sheet or due to paper dust. This advantage assures high reliability. Such transfer type ink jet printers are disclosed in U.S. Pat. Nos. 4,538,156 and 5,099,256.
In the disclosed apparatuses, an ink jet recording head (hereinafter, referred to as a recording head) having a plurality of nozzles is separated from a cylindrical transfer medium by a gap. When an ink image is to be formed on the transfer medium by the recording head, the recording head forms an ink image on the cylinder in accordance with an image signal in synchronization with the rotation of the cylindrical transfer medium, while the recording head moves in a direction parallel to the direction in which the nozzles are arranged. Then a recording medium is brought into contact with the transfer medium and pressed from the back against the transfer medium, whereby the ink image is transferred to the recording medium. Thereafter, the recording medium is discharged from the apparatus.
In another known method (Japanese Patent Publication (Kokai) No. HEI4-169,236), in order to dry and fix a recorded ink image to improve the printing quality, the total number of printed dots or the density of printed dots is counted, and a drying heater is controlled in accordance with the counted value.
In any of the above apparatuses, the transfer step must be conducted at a low pressure to ensure high efficiency and complete transfer of the image. Specifically, the step of transferring an ink image from a transfer medium to a recording sheet is conducted by applying a pressure, and therefore the transfer medium must be made of a material from which the ink image is easily peeled or which has a low surface energy. On the other hand, the ink image on the transfer medium is formed by dots produced by ink drops ejected from the recording head. Because of surface tension, such dots have a tendency to gather on a material having a low surface energy. Therefore, plural dots aggregate on the transfer medium to form a large dot. As a result, a dot may be formed at a position different from where the dot should be formed, or an ink image formed by an aggregation of dots may be deformed. Hereinafter, this phenomenon is referred to as the repellent phenomenon.
FIG. 1 schematically shows a typical example of the repellent phenomenon observed when an image is formed by groups of dots. In a prior art apparatus, a recording method is employed in which a recording head 101 having a plurality of nozzles 110 is moved in a subscanning direction indicated by an arrow U in the figure, so that dots are sequentially written from the initial position indicated by S1 to a position indicated by S2. According to this method, ink dot strings 103 are continuously sequentially overlapped on a transfer medium 102 starting from the side where the writing is initiated. The repellent phenomenon occurs more easily as the quantity of overlapping ink drops increases or as the ink amount per unit area increases. Moreover, the drying of the ink dot strings proceeds starting from the side where the writing is initiated. In an area 104 close to an ink dot string written in an earlier stage, therefore, the repellent phenomenon does not occur. In an area 105 where ink dot strings are written and the printing duty is high, conversely, there may arise a case where the repellent phenomenon occurs so that the area is divided into portions 106 having ink and portions 107 having no ink.
Japanese Patent Publication (Kokuku) No. HEI4-19,030 discloses a recording method in which ink drops are directly impacted to a recording medium while ink dot strings are alternately written in forward and reverse paths on every other row in a direction perpendicular to a scanning direction of a recording head. When this recording method is applied to a transfer type ink jet recording apparatus, however, the transfer medium does not absorb water contained in the ink as does a recording sheet, so that before an ink dot string written in the immediately preceding scanning step is completely dried, an adjacent ink dot string is written. Therefore, the repellent phenomenon occurs on the transfer medium in the manner described above, resulting in an image having reduced quality.
Also, in transfer type ink jet recording apparatuses having the above configurations, an ink image is formed while the cylindrical transfer medium makes plural turns. Therefore, the time required for forming a film of the ink image on the transfer medium varies so that the conditions of the film formation become uneven. This causes a transfer stain to be produced on a recording sheet, or causes the ink image on the transfer medium to remain partly untransferred, so that a high quality recording image cannot be obtained.
In the case where the film-forming conditions are corrected by controlling a heater or the like in accordance with the density of printed dots, furthermore, it is impossible to control the energy applied to each dot, and therefore the correction cannot be conducted in a perfect manner.
The invention has been conducted in view of the above-discussed problems. It is an object of the invention to provide an ink jet recording method and apparatus in which the repellent phenomenon that may produce an image of unacceptable quality is prevented from occurring on a transfer medium which conducts a low pressure transfer and has a high peeling property. Another object of this invention is to prevent variations in conditions under which a film is formed on a transfer medium from reducing the quality of a recording image formed on a recording medium.